Tools using brushes are inoperable.

My tools that use a brush (ex. clone stamp, burn/dodge, etc..) don't work, while my tools that don't use a brush (ex. crop, move, etc..) do work. When I use one of the seemingly inoperable tools, I see a record of the tool step in the history palette, but nothing happens to my pixels. I have reset all preferences to no effect. This occurs on all types of files, including background layers.

What has happened to me? Any help would be appreciated.

Thank you.

un-check brush dynamics in the brushes palette. if you have a tablet, make sure the pen isn't resting on the tablet when you're using a mouse.